Modern automotive ignition systems have provided protection from fault conditions that result in excessively long dwell times by a variety of techniques. For example, some systems have employed time-based protection that terminates a drive signal to an ignition coil current switching device, e.g., an insulated gate bipolar transistor (IGBT), when a predetermined dwell time was exceeded. Other existing systems have employed thermally triggered dwell termination that functions when excessively high temperatures are detected within a switching circuit and/or a control integrated circuit (IC). These methodologies, while generally effective in preventing damage to a control IC and/or a switching circuit, have several drawbacks. For example, time-based protection methods have required external timing capacitors.
Further, current thermal shutdown techniques generate a spark event at the spark plug, following the thermal shutdown, unless an external capacitor is utilized for control purposes. This is undesirable as a mistimed spark can cause damage to an engine if the spark leads or lags a desired spark timing point by any appreciable amount. Further, when the protection mechanism is purely thermally based, a number of variables, e.g., battery voltage, ignition coil resistance and ambient temperature of the control electronics, affect the timing of a thermal shutdown. As such, it is difficult to predict an absolute time, following the start of dwell, that a spark may occur. It should be appreciated that protection of an ignition system at the cost of engine damage is undesirable.
Some existing over-dwell protection systems have decreased an ignition coil current in a manner that has prevented the production of a spark at an associated spark plug. This type of sparkless dwell termination is commonly referred to as a “soft shutdown.” An example of one such over-dwell protection system is disclosed in U.S. Pat. No. 5,819,713. This over-dwell protection system uses external timing capacitors to generate a slow ramp down voltage which is, in turn, applied to a control terminal of a switching device, e.g., a gate of an IGBT. While accomplishing the task of terminating a dwell event without the production of a spark, such systems incur the cost of an external capacitor, increased packaging space required for the capacitor and related interconnect wiring, as well as additional control die area for the required soft shutdown circuitry.
Advances in electronic packaging techniques are allowing modern ignition control electronics to be packaged with increasing density and to be placed into hostile environments. In many cases, integrated electronic control circuits are being packaged with the high power switching transistors that they control, in single over-molded plastic packages. These packages are then embedded in individual ignition coil housings. With these types of assemblies, package size, interconnect reliability and cost may result in significant design constraints. The additional control die area and expense of external capacitors and the related interconnects required for the existing soft shutdown protection implementations are therefore undesirable.
What is needed is a technique for performing temperature-based over-dwell termination that provides a soft shutdown without requiring external timing or soft shutdown control capacitors.